1. Field of the Invention
The present invention relates to a polymeric optical device and a method of fabricating the same, and more particularly, to a polymeric optical device and a method of fabricating the same capable of reducing variations of wavelength characteristics depending on polarization.
2. Discussion of Related Art
A polymeric optical device has a merit of easily being fabricated, but it has a limitation in an application thereof due to high polarization dependence as compared with the silica optical device, particularly, in the case of an arrayed waveguide grating (hereinafter, referred to as AWG) device. In case of an AWG fabricated by a silica material, a central wavelength difference according to a TE/TM polarization mode is about 0.35 nm (See H. Takahashi, Y. Hibino, and I. Niship, “polarization-insensitive arrayed-waveguide grating wavelength multiplexer on silicon”, Optics Lett., vol.17, no.7, pp499–501, 1992). In the case of an AWG device using a polymer, the central wavelength difference according to the TE/TM polarization mode reaches a few nm and it becomes ten times larger than that of the silica AWG device (See Y. H. Min, M. H. Lee, J. J. Ju, S. K. Park, and J. Y. Do, “polymeric 16×16 Arrayed-Wavegudide Grating Router Using Fluorinated Polyethers Operating Around 1550 nm”, IEEE J. Select. Topics in Quantum Electron., vol.7, no.5, 806–811, 2001).
Although the central wavelength difference of the silica AWG device is as low as about 0.35 nm, it cannot be ignored. In order to solve this, a method of reducing the TE/TM shift by inserting a half-wave plate into a grating channel portion in the AWG device or coating an amorphous silicon layer (See H. Takahashi, Y. Hibino, Y. Ohmori, and M. Kawachi, “Polarization-insensitive Arrayed-Waveguide Wavelength Multiplexer with Birefringence Compensating Film”, IEEE Photon. Technol. Lett., vol.5, no.6, 707–709, 1993), a material having a specific thermal expansion coefficient (See S. M. Ojha, C. Cureton, T. Bricheno, S. Day, D. Moule, A. J. Bell, and J. Tayleor, “Simple method of fabricating polarization-insensitive and very low crosstalk AWG grating device”, Electron. Lett., vol.34, no.1, 78–79, 1998), or a material having a specific birefringence characteristic (See S. M. Ojha, et al., “Planar Optical waveguide”, U.S. Pat. No. 5,930,430, Jul. 27, 1999), which can offset a stress due to a substrate, on the grating of the AWG device, has been used. By adopting the aforementioned method, the central wavelength shift of the TE/TM in the silica AWG device could be reduced by 0.1 nm or less.
On the other hand, in the case of a polymeric optical device, the TE-TM shift is as large as a few nm due to the stress effect generated by the birefringence of the polymer itself and fabrication thereof. As for a method of reducing the TE-TM shift, most studies have been emphasized on developing a material having low birefringence, by varying characteristics of the polymer itself (See Y. Koike, et al., “Process for the preparation of non-birefringent optical resin and optical elements made by using the resin prepared by the process”, U.S. Pat. No. 6,277,938, Aug. 21, 2001). However, since most of the birefringence in the polymeric optical device is caused by the stress effect on the fabrication thereof, there is a burden of developing a new material. Meanwhile, there is a difficulty in fabricating a polarization-insensitive AWG device, in the case of fabricating an optical device with a limited material.
As for a method of reducing the stress of the polymeric optical device, a method for replacing a conventional silicon wafer of a polymeric optical substrate with a polymer has been proposed (See N. Keil, H. H. Yao, C. Zawadzki, J. Bauer, M. Bauer, C. Dreyer, and J. Schneider, “A thermal polarization-independent all-polymer arrayed waveguide grating (AWG) multi/demultiplexer”, OFC 2001, Anaheim, post-deadline paper PD7, 2001). But, in this case, since a specific polymer should be used as the substrate, it is difficult to fabricate it and to pack it with a different element.